Circuit breakers are widely used to protect electrical lines and equipment. The circuit breaker monitors current through an electrical conductor and trips to interrupt the current if certain criteria are met. One such criterion is the maximum continuous current permitted in the protected circuit. The maximum continuous current the circuit breaker is designed to carry is known as the frame rating. However, the breaker can be used to protect circuits in which the maximum continuous current is less than the circuit breaker frame rating, in which case the circuit breaker is configured to trip if the current exceeds the maximum continuous current established for the particular circuit in which it is used. This is known as the circuit breaker current rating. Obviously, the circuit breaker current rating can be less than but cannot exceed the frame rating.
An electronic trip unit (xe2x80x9cETUxe2x80x9d) is a device that is used in conjunction with an electro-mechanical circuit breaker to control the current verses time trip response. The time versus current trip characteristics are, in part, a function of the maximum continuous current permitted by the circuit breaker. This maximum continuous current is also called the current rating of the circuit breaker. As long as the current remains below this maximum continuous current rating, the breaker will remain closed. Momentary low magnitude excursions above the rated current are tolerated; however, persistent overcurrents result in tripping of the breaker. The time delay and generation of the trip signal is an inverse function of the magnitude of the current. For very large magnitude overcurrents, such as would be produced by a fault, the microcomputer is programmed to generate a trip signal instantaneously.
The modification of the Current vs. Trip time response curve is a serious matter. For safety purposes, the circuit breaker must be properly configured to provide the type of protection judged by the customer or plant engineer to be appropriate. Therefore the modification to this protection must also be considered to be a very serious event and handled in a way that prohibits errors.
Typically, the circuit breaker current rating is set by a rating resistor (a xe2x80x9cburden resistorxe2x80x9d) which is selected to generate a preset voltage when a current proportional to the maximum continuous current permitted in the protected circuit passes through the rating resistor. In order to provide for adjustment of the current rating so that the circuit breaker can be used to protect circuits with different maximum continuous currents, it is known to incorporate the rating resistor in a replaceable rating plug which may be selectively inserted into the breaker.
Electronic trip circuit interrupters are designed to interrupt overcurrent conditions over a wide range of ampere ratings. The current through the protected electric power circuit is continuously sensed by means of current transformers and a voltage signal is supplied to the signal processor within the ETU circuit by means of so-called xe2x80x9cburden resistorsxe2x80x9d, such as rating resistors in a rating plug. The size of the burden resistor accordingly sets the ampere rating of the corresponding circuit interrupter. A common electronic circuit interrupter could therefore operate over a wide range of ampere ratings by merely changing the value of the burden resistor within the electronic trip circuit. It is important to prevent an electronic circuit interrupter from being inserted within an electrical distribution circuit for which the circuit interrupter is over-rated. It is perhaps equally important not to insert a circuit interrupter within an electric power distribution circuit for which the circuit interrupter is under-rated, as so-called xe2x80x9cnuisance-trippingxe2x80x9d could occur. It is also important to insure that a circuit interrupter is not inserted within an electric power distribution circuit with no rating plug or burden resistor whatsoever.
Field replaceable rating plugs are known. These plugs are field installable and may be mechanical for use with thermal-magnetic trip units or may use a combination of analog circuit scaling and digital techniques to change the ETU response. It is typical for these plugs to provide mechanical rejection of plugs that are not suited to certain ranges or frame sizes.
A typical method to prevent incompatible ETU/rating plug combinations includes a first manufacturing process of providing interlocking pins that can be mechanically modified by a secondary manufacturing process of breaking out pieces. The secondary manufacturing process breaks out small pieces of plastic on the housing of the rating plug and complementary pieces on the housing of the ETU.
A problem associated with mechanical rejection of plugs is costs associated with the secondary operation and the limitation of the number of combinations that can be rejected. In some cases the mechanical rejection method is not reliable because some operators, using great force, can insert an incorrect rating plug.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method and apparatus for rejection of an inappropriate rating plug in an electronic trip unit for a circuit breaker including; a microprocessor programmed to determine an overcurrent condition of the circuit breaker; a nonvolatile memory in operable communication with the microprocessor; the rating plug releasably engaged with the microprocessor, the rating plug includes an identification register; wherein the microprocessor reads the identification register, the identification register includes an identification number; wherein the microprocessor accesses one of a plurality of programs in the nonvolatile memory based on the identification number; and wherein the one of a plurality of programs instructs said microprocessor to perform a validation of said rating plug for operation with said microprocessor.
The above discussed and other features and advantages of the present disclosure will be appreciated and understood by those skilled in the art from the following detailed description and drawings.